Fix Collections have missing items in certain conditions

[Gio2018]

Goal

• Fix an issue where some collections inside collections did not show as such, and were opened in a web view. That was caused by associated Items to be purged unintentionally

Test Steps

• Pick a collection (e.g. https://getpocket.com/collections/making-the-most-of-mental-health-awareness-month)
• Make sure the collections inside the collection show the "Collection" label and open in the native collections view
• Wait several minutes, refresh and make sure the above steps still succeed.

[pocket-ci]

	Messages
📖	No SwiftLint violations! 🎉
📖	Project coverage: 38.59%
📖	Checking XCode Environment Variables
📖	Edited 3 files
📖	Created 0 files

PocketKit: Coverage: 60.44

File	Coverage
CollectionStory+Extensions.swift	0.0%	⚠️
CollectionStoryViewModel.swift	9.28%	⚠️

Sync: Coverage: 69.96

File	Coverage
Space.swift	72.51%	✅

SyncTests: Coverage: 22.82

File	Coverage
Space.swift	57.32%	✅

PocketKitTests: Coverage: 28.42

File	Coverage
CollectionStory+Extensions.swift	0.0%	⚠️
CollectionStoryViewModel.swift	0.0%	⚠️

Generated by 🚫 Danger Swift against 5620501